Conventionally, synthetic fibers including fibers of polyesters such as polyethylene terephthalate (PET), polyamide fibers such as nylon, and polyolefin fibers such as polyethylene and polypropylene generally are produced by a melt-spinning method. Ultrafine fibers such as nanofibers are known to be formed by splitting a sea-island fiber into nanofibers by removing the sea component of the sea-island fiber with the use of a solvent so as to leave the island component that will be nanofibers (Patent Document 1). Such ultrafine fibers have a small fiber diameter, and thus thermally-bonded non-woven fabrics made of the ultrafine fibers are suitable for use as clothing materials, automobile components and the like. For example, Patent Document 2 discloses a thermally-bonded non-woven fabric made of nanofibers, and proposes production of the non-woven fabric by desorbing and removing the sea component from a sea-island fiber having two different types of island components and thermally bonding the two types of island components. This method, however, is disadvantageous in that it requires removal of a large amount of sea component by using a solvent after spinning or production of the non-woven fabric, which makes the process complex and costly.
Another known method for producing an ultrafine fiber is an electrospinning method as described in, for example, Patent Document 3 in which an ultrafine fiber is spun by applying high voltage to a polymer solution or a polymer melt. The electrospinning method is said to be cost advantageous in that it does not require removal or disposal of the sea component and is simple. It is known that the electrospinning method can be classified into a solution method and a melt method, depending on the system for supplying a source material. The solution method is a method including supplying an aqueous solution containing a source resin dispersed therein or a flowable source resin solution, electrifying the source resin to charge the source resin, and forming a fiber by electrical attraction. On the other hand, the melt method is a method including supplying a solid source resin-formed product, electrifying the source resin to charge the source resin, heating and melting the source resin, and forming a fiber by electrical attraction.
Conventional battery electrode plates and separators are produced separately, and a plurality of electrode plates and a separator are laminated and housed in a battery for use. Merely laminating (without fixing) the electrode plates and the separator is problematic in that, since the separator itself is thin, the electrode plates and the separator may be displaced relative to each other or wrinkled during the process of housing them in a battery. In addition, in order to reduce such displacement and wrinkling, it is necessary to set the line tension during the housing process so as to correspond to the separator having a low tensile strength, and this causes a problem of slowing down the production.
Furthermore, integrated configurations of an electrode plate and a separator have been proposed (Patent Documents 4 and 5), but such configurations are obtained by separately producing the electrode plate and the separator, and then integrating them by using an adhesive or by heat treatment. That is, there is a process of affixing the electrode plate and the separator, and inhibition of displacement and wrinkling has not been achieved. In addition, with such integrated configurations, the electrode plate and the separator will be detached from each other easily because they are separately produced and then integrated together.
Recently, attempts have been made to produce a battery separator by using a solution electrospinning method (Patent Document 6), but this technique is employed only to process a single component polymer into a fiber to give a separator, and such separators have a problem of undergoing significant shrinkage during heat treatment. Also, there is another problem: in the solution electrospinning method, in particular, the solvent or water used during the spinning process remains in the fiber, which may have harmful effects on the product.